A composite film having an acrylic-based polymer and a urethane polymer has been known as a film having both high strength and high elongation at break. In JP 2001-520127 W, a multi-layered film comprising an interpenetrating polymer network layer (IPN layer), and at least one fluorine-containing polymer layer is disclosed as a surface protection film of automobiles and the like. The IPN layer in the multi-layered film makes use of an IPN composite of a urethane polymer and an acrylic polymer, and is prepared by applying a mixture of an acrylic monomer, an acrylic cross-linking agent, and a urethane cross-linked precursor of a polyol and polyisocyanate to a base material, polymerizing and cross-linking the acrylic monomer and the polyol/polyisocianate being the urethane precursor with heating in the manner of nonintervention, each.
According to this method, there is a merit that limitation (restriction) of types, combinations and blending proportions of the monomers to be used is difficult to be generated, but there is a problem in productivity because the urethane polymerization is a polyaddition reaction which proceeds slowly in comparison with such a chain reaction as the acryl.
In order to solve the problem as to productivity, when an IPN layer was tried to be obtained by utilizing the sequential synthesis and photopolymerization as disclosed in JP 2003-96140 A, the cross-linked urethane polymer became swelled in the presence of the acrylic monomer and the cross-linking agent to thereby increase a viscosity of the syrup remarkably, and accordingly, there arose a problem that application to a base material by coating or casting was very difficult.
In addition, when cars drive, there is a case where powder dusts, pebbles, and the like clash to a coating surface of body. Particularly when cars drive on a rough road or roads where rock salt, sand, gravel and the like are sprayed for removing snow or the like in cold districts, there is a problem that the coating surface tends to be damaged and rust is generated from the damaged portion of the coating surface. In automobile fields, in order to prevent the coating surface of an automobile body from damage, a transparent adhesive tape may be applied thereto. A polyurethane base material is used as a base material of that transparent adhesive tape (for instance, refer to JP 59-41376 A, JP 2005-272558 A).
However, it is known that a polyurethane generates a coloring substance exhibiting conjugate structure, and a nitrogen-containing coloring substance through photoreaction. Therefore a film containing a polyurethane is initially colorless and transparent, but when left outdoors, there is a case where the film turns yellow because of being exposed to ultraviolet rays and its glossy feeling disappears to reduce good appearance.
Although an adhesive sheet in which a coating layer composed of a fluororesin is provided on a base material is also known, adhesion strength between the base material and the coating layer is not sufficient.